Ocean Cooling, Not Pollution, Halted Global Warming in Mid-20th Century

A new paper in the journal Nature explains what happened during the mid-twentieth century to halt the ever-increasing global temperature rise that continues to this day. Rather than warming in the Northern Hemisphere being stopped by a greater build-up of air pollution as had been supposed, the researchers say an unexpectedly abrupt cooling of oceans between 1968 and 1972 put warming on hiatus. David Thompson of Colorado State University says,

The suddenness of the drop in Northern Hemisphere ocean temperatures relative to the Southern Hemisphere is difficult to reconcile with the relatively slow buildup of tropospheric aerosols. We don't know why the Northern Hemisphere ocean areas cooled so rapidly around 1970, but the cooling appears to be largest in a climatically important region of the ocean.

Though the data shows temperature drops throughout Northern Hemisphere oceans, the most pronounced declines occurred in the North Atlantic.

The reason aerosols were previously suspected is that some types of aerosol pollution (sulphates from coal burning being an example) reflect sunlight, counteracting the greenhouse effect and masking warming that would otherwise happen due to greenhouse gas accumulation alone.